Demountable Superconducting Magnet Coils

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1 FESAC TEC Report 1 Demountable Superconducting Magnet Coils A strategic technology to address key nuclear materials, construction, and maintenance issues Brandon Sorbom, Bob Mumgaard, Joseph Minervini, Makoto Takayasu, Dennis Whyte

2 HTS enables demountable joints in the superconducting TF coil, greatly simplifying maintenance and upgrade. High availability and upgradability is key to an economic fusion reactor We need to test configurations and materials but cannot build an entire new device for every test, would like to test in the same device. Termed a Fusion Nuclear Science Facility (FNSF), reconfigurable to develop the required techniques in a staged approach. Similar to how the first nuclear reactors worked (e.g. Shippingport), testing different materials and cores. Traditionally, the superconducting TF acted like a cage around the internal components making upgrade or replacement difficult Hard to install demountable joints in LTS due to resistance of normal materials causing local heating leading to quench. Elaborate schemes for maintenance have been envisioned to remove components from inside through gaps between enlarged TF coils. Leads to requirements to align and seal many large or multitudes of small components using remote handling robots during an outage. Demountable HTS joints enable modular maintenance Enabled by higher temperature operation, several designs under test around the world. ARC s TF top half is removable in a single large lift (still less than one ITER TF coil, small and modular are mutually reinforcing). Can then remove vacuum vessel in single piece and place in hot cell. New vacuum vessels with all internal components installed and tested can then be installed and operation resumed. Sector maintenance: The standard, where the fusion blanket is cut into sections in place, removed through the TF magnet in many pieces then new pieces are inserted one by one and re-welded. Vertical maintenance: Enabled by joints, the TF cage is removed and only the vacuum vessel is removed in one piece, swapped with a ready-made replacement. FESAC TEC Report 2 Upper TF Liquid blanket tank Lower TF 2 Vacuum vessel containing complex internals

3 FESAC TEC Report 3 Problem: Complex maintenance schemes require long-lifetime internal components to be economical and this slows development Nb 3 Sn low-temperature superconductor coils cannot be split No temperature margin operating at 4K This lead to necessity that interior nuclear component must be sectored and be able to fit in space between the coils ~20 sectors need to be replaced to exchange the neutronexposed internal components The sectors must be inserted and connected inside the reactor building a ship in a bottle. Alignment must be precise to prevent melting from plasma exposure Slow and risky (> 100 s welds inside reactor!)...iter takes three years to change its interior components This forces solid materials to last longer..thus material must tolerate very high dpa and He accumualtions Key to economics becomes ever more resilent materials Development path has long timescales for iteration of materials Sector maintenance relied upon with LTS magnets

4 Solution: HTS enables jointed magnetic field coils, enabling modular maintenance at regular intervals while experience is gained. FESAC TEC Report 4 ARC 250 MWe Pilot plant B. Sorbom et al Fusion Eng Design 2015 Demounted TF coils using HTS HTS enables joints in coils Joints have finite resistance, but the ohmic heat can be taken out due to higher temperature operation window of the HTS. The HTS requires no in-situ heat treatment so can be mounted and demounted many times as long as the joint is structuraly sound. The ARC conceptual design + subsequent PhD thesis shows there are several pathways to make these joints viable. The internal components can be extracted as single toroidal units Replacement units can be ready to be installed in short outages. Enables the expensive parts of the design such as TF coils to be used to test multiple concepts for neutronexposed components. Lessens incentive to rely on very long lived components.

5 Problem: Internal sectored blankets force daunting engineering design challenges. This has long been recognized by the fusion community... FESAC TEC Report 5 Because it cannot be welded inside, the vacuum vessel (VV) must be a lifetime components, outside the blanket. The blanket must be hung off the VV with integrated cooling channels. The blanket must be structurally strong more steel poor neutron thermalization thicker blanket bigger tokamak The blanket must work in a vacuum! Any coolant leak stops fusion operation Installation and repair done in an activated environment Any gap between sectors will leak neutrons to VV and sensitive superconducting coils There s a good reason fission system experts dismiss fusion s near-term viability if it looks this complex and sensitive!

6 Solution: Joints combined with a liquid immersion blanket to simplify fusion nuclear technology and deal with neutrons Move VV right up beside plasma because the plasma is the only thing that needs vacuum! VV becomes a replaceable component instead of a lifetime component Can be replaced as a single component in demounted coil geometry Immerse VV (+ plasma) in a tub filled with a low-z liquid This liquid becomes the blanket to primarily interact with neutrons Carries out fusion heat No damage limits in liquid Liquid has no cracks or gaps Operates at atmospheric pressure Cools the vacuum vessel Supports the vacuum vessel Primary confinement now becomes permanent outer blanket tank Shielded by liquid blanket Immersion Blanket Blanket tank Sector Blanket Demountable coils allow us to implement our idealized design choices for neutron interactions (slide #11) FESAC TEC Report 6

7 Joints, when coupled with a liquid molten salt blanket, allow for a new paradigm for handling nuclear challenges FESAC TEC Report 7 Don t rely on discovering physics miracles Waiting for unobtainium that can resist all neutron damage is not a strategy. Non D-T fusion at high gain requires a leap of faith in plasma physics. Have an R&D strategy that avoids the biggest known pitfalls of neutron damage primarily through technology innovations integrated into the designs Fusion neutrons damage solid material So minimize solid materials that interact with neutrons! But still realize you must replace some materials design them to be replaceable from the start, not as an afterthought Take on risks when you have to, and then retire them early and at low cost Recognize that the nuclear issues can be separated from the plasma physics issues. SPARC avoids n-induced degradation by design; it settles the key question of energy gain and selfheating with short duration (~10 s), but equilibrated plasmas due to its very small size neutron fluence and damage are not a concern. Solve the nuclear issues in flexible devices while you provide fusion power ARC pilot plant with demountable coils and a liquid blanket is tailored to prototype multiple configurations and materials in an adaptable and expedited manner This strategy anticipates there will be unknown unknowns.

8 Joints coupled with liquid blankets allow PF coils internal to the TF coils, enabling innovative divertor configurations Low-pressure Liquid tank Topology and construction of non-jointed, LTS TF forces PF coils to be installed outside of the TF coils, far from the plasma Forces PF coils to be stronger than necessary Makes innovative divertor configurations very difficult or impossible Jointed TF coils allow PF coils to be moved inside of the vacuum vessel PF coil size can be reduced Proximity to vacuum vessel allows novel divertor approaches, such as X- point or long-leg divertors Clever geometric arrangement of divertor high-heat flux region allows it to be well-shielded from neutrons Separation of functionality, ultimately enabled by superconducting joints Follow-on ARC work (in process of being published) indicates that long-leg divertor design enabled by joints has several advantages Less than 5 dpa/year to divertor region increases structural material survivability Reduced heat fluxes Neutron field Magnetic field ARC Interior Fusion plasma Long leg divertor Liquid HTS Coils that shape plasma shielded from neutrons Vacuum vessel FESAC TEC Report 8

9 HTS joint technology at TRL 4 Benchtop experiments and medium scale testing combined with design scoping studies FESAC TEC Report 9 30 ka-class GdBCO joint 80-tape REBCO demountable joint Applying contact pressure Mechanical triple dovetail linkages ARIES-class demountable coil set ARC demountable coil set NIFS Joint Testing and Conceptual Designs MIT Joint Testing and Conceptual Designs

10 There is already extensive experience in vertical maintenance of demountable copper magnet tokamaks: Alcator C-Mod and DIIID C-Mod maintenance enabled by high-field demountable cryogenic copper magnets. Transition from DIII to DIIID vacuum vessel aided by jointed coils The demountable coils of Alcator C-Mod. Installing the toroidally continuous modular vacuum vessel. This paradigm looks similar to fission experience. FESAC TEC Report 10

11 Critical variable for HTS joints is achievable joint resistance and mechanical stability at fusion-relevant scale FESAC TEC Report 11 Concrete first metric of success: Operate a superconducting TF coil at full field without performance degradation after demounting and re-assembling the coil a certain number of times Performance metrics are joint resistance and mechanical integrity of structure Joint resistance determines economics of joint use in pilot plant: higher resistance = joule heating = larger cryoplant = $$$ Mechanical integrity determines feasibility of using joints on reactor-scale device with large forces due to high fields b Reactor-relevant joint resistances achieved at MIT in benchtop demountable joint experiments and in Japan in 100 ka-class joint Large-scale mechanical testing still required, although aforementioned experience with copper joints

12 No basic physics showstoppers largest risk is whether engineered joints can be economically viable FESAC TEC Report 12 Main uncertainty simply due to lack of data as a result of low funding and a limited test program Economic studies of fusion reactors have high uncertainty, making economic assessments of joints speculative at this point While there is little doubt that superconducting joints could be built, a balance between three engineered parameters is needed: Low joint resistance leading to low heat generation Simple mechanical joint design to enable fast maintenance Robust joint design to prevent operational damage No inherent safety issues with this technology (typical industrial safety protocols applicable) b

13 HTS joint research represents an excellent opportunity for US investment Limited research, both in the US and around the world (NIFS in Japan is largest outside research group) Would most likely fall under Enabling R&D in FES budget, which is being cut by ~38% in 2018 Strong base of US superconducting development (e.g. National High-field Magnet Lab, MIT, Tufts, LBNL) to support joint research for fusion Joint technology would be patentable, attracting outside private investment at later stages to bridge technological valley of death b FESAC TEC Report 13

14 FESAC TEC Report 14 References [1] Sorbom, B. N., et al. "ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets." Fusion Engineering and Design 100 (2015): [2] Mangiarotti, F. J., & Minervini, J. V. (2015). Advances on the design of demountable toroidal field coils with REBCO superconductors for an ARIES-I class fusion reactor. IEEE Transactions on Applied Superconductivity, 25(3). [3] Mangiarotti, Franco Julio. Design of demountable toroidal field coils with REBCO superconductors for a fusion reactor. Diss. Massachusetts Institute of Technology, [4] Nishio, T., Ito, S., & Hashizume, H. (2017). Heating and Loading Process Improvement for Indium Inserted Mechanical Lap Joint of REBCO Tapes, 27(4). [5] Yanagi, N., Ito, S., Terazaki, Y., Seino, Y., Hamaguchi, S., Tamura, H., Sagara, A. (2015). Design and development of high-temperature superconducting magnet system with joint-winding for the helical fusion reactor. Nuclear bfusion, 55(5), [6] Ito, S. (2015). Mechanical and Electrical Characteristic of a Bridge-type Mechanical Lap Joint of HTS STARS Conductor, 26(2). [7] Ito, S., Seino, Y., Yanagi, N., Terazaki, Y., Sagara, A., & Hashizume, H. (2014). Bridge-Type Mechanical Lap Joint of a 100 ka-class HTS Conductor having Stacks of GdBCO Tapes, 9( ).